Miniaturization of mechanical devices at the micro- and nanoscale, referred to as micro- and nanomechanical systems (MEMS and NEMS, Microelectromechanical Systems and Nanoelectromechanical Systems, respectively), has allowed the development of advanced scanning technologies of mechanical sensors and is quite relevant in the fields of electronics and power generation. The potential of these systems is based on the fact that displacement on the nanometric scale and vibration of such system is very sensitive to external forces, farces generated on the surface plane thereof and the mass added thereon. These attributes mean that micro- and nanomechanical systems combined with optical or electric displacement sensors can be applied in detecting the force between two molecules or atoms, with a sensitivity in the attonewton range, and for measuring a deposited mass with a sensitivity in the zeptogram range, and at the same time finding the elastic constant, with kilopascal resolution; or for detection applications without surface molecular markers through the generated forces. MEMS and NEMS have also been used as high-precision radio frequency filters, accelerometers and gyroscopes.
In the current state of the art, cantilever-based micro- and nanomechanical systems are known to have a fixed end and a mobile end. The placement and/or motion of the free end are what are normally detected in these systems. Cantilever-based systems fixed at both ends, of the type in which the motion of the central part thereof can be detected are also well-known.
Hereinafter, when discussing the general background of the invention, reference will be made to cantilevers having a fixed system and a free system, the deflection of which must be measured (deflection refers to displacement of the free end of the microcantilever) in response to light, but advancements in such architectures are also applicable to other previously mentioned designs.
When cantilevers are reproduced at the microscale, conventionally with a thickness of 0.2-1 μm, width between 100-500 μm and length of 100-500 μm; such cantilevers can bend in the order of a few nanometers in response to forces in the piconewton range. It is in this range where forces between atoms, molecules and biomolecules governing many of the physicochemical properties of materials, as well as many fundamental life processes, converge. The corresponding deflections of microcantilevers can be optically and electrically detected with a resolution of at least 100 pm/Hz1/2.
Generally, there are various techniques for reading the deflection of the cantilever, such as capacitive sensing, tunneling current sensing, optical interferometry, piezoresistive reading, as well as the so-called optical beam deflection techniques. The latter is the most widely used method due to its simplicity, enormous sensitivity and its ability to measure in air, mixtures of gases and fluids without contacting displacement sensors or the reading circuitry.
As previously mentioned, the optical beam deflection method is very sensitive and has the advantage that it can be readily implemented. A segmented photodetector split into two segments oriented parallel to the axis of motion of the cantilever is normally used for capturing the reflected beam. Deflection of the cantilever causes displacement of the laser dot reflected on the photodetector. The difference in photocurrents between the two segments is therefore proportional to the deflection of the cantilever.
In addition to static deformation, resonance frequencies of micro- and nanostructures for soft surface scanning in AFM and for the development of sensors which are based on the addition of masses and the rigidity of the molecules captured on the surface of the cantilevers have been measured. Cantilevers are usually considered to be structures having a unique resonance frequency (fundamental resonance frequency), exciting them to frequencies close to the resonance frequency; however mechanical structures have several modes of vibration at higher frequencies than the fundamental frequency.
There is growing interest in the use of high frequency modes to increase sensitivity and detection limits. However, it is extremely complex to determine the mode shape at frequencies corresponding to resonance frequencies, which is very important for quantifying and interpreting the measurements. Knowledge about the shape of modes of vibration and the accurate measurement of corresponding frequencies is extremely relevant for the design of MEMS and NEMS in all fields of application.
There are also issues concerning the dynamic behavior of cantilevers that have not been resolved, such as: the effect of the surface stress on resonance properties or coupling between modes of vibration induced by viscous damping, elastic elements or by intermittent contact. The emergence of finite element simulations and the ever-increasing computer processing speed shed light on these issues. However, these simulations are very time-consuming and ignore defects and flaws inherent to micro and nano manufacturing processes. On the other hand, the free parameters in the simulation, such as grid size or the definition of contour conditions and pre-stressing conditions of the structure cannot always be chosen or determined in a realistic manner, so simulations can only serve as a guide in the design of MEMS and NEMS but not as a tool capable of realistically simulating the behavior of these structures. The experimental measurement of the shape of modes of vibration and the determination of their frequencies is a critically necessary tool; this tool is the object of the present invention.
Recently, scanning Doppler laser vibrometry (SDLV) [Biedermann L B et al. (2009) “Flexural vibration spectra of carbon nanotubes measured using laser Doppler vibrometry”] and phase-shifting interferometry [Kelling S. et al. (2009) Simultaneous readout of multiple microcantilever arrays with phase-shifting interferometric microscopy”] (WLI, White Light Interferometry) have demonstrated a significant ability for characterization of nanomechanical systems. SDLV can obtain images with high sensitivity with respect to vibration, outside the plane of these systems in the sub-angstrom range and with submicrometric lateral resolution. WLI provides information about topography with a vertical resolution of 1-10 nm. Additionally, implementation of the stroboscopic lighting in WLI has allowed mode of vibration analysis, although the process is slow, bandwidth is limited and resolution is still insufficient.
Therefore, despite the existing state of the art techniques that can provide simultaneous information about the static and dynamic behavior of nanomechanical systems with high sensitivity in a rapid and simple manner are still needed.
Specifically, the most widely used systems today for dynamic motion/displacement analysis, WU LDVP (Laser Doppler Vibrometer) cannot simultaneously obtain information about the motion of the micro- or nanostructure, static deformation and motion at various vibration frequencies, more scans being necessary, one in each frequency scenario.
Additionally, although these techniques may be practical for measuring the motion/displacement of individual elements, many practical uses of the systems for measuring micro- and nanomechanical elements require the use of arrays having a large number of micromechanical elements comprising a plurality of cantilevers arranged in a certain formation and operating in parallel, therefore providing greater speed and multifunctionality. This invention proposes that laser beam deflection systems are suitable for measuring both static and dynamic behavior of elements/cantilevers, for example: maximum deflection, mean deflection value, the amplitude at a reference frequency (the element can be excited externally by means of an excitation force that oscillates at a reference frequency), a phase of motion with respect to an excitation signal, a frequency, etc. Measured static displacement, amplitude, frequency, etc., can be related to an object that must be measured and interacts with the cantilever, and to the signals used to stimulate the object and/or cantilever.
Although the optical beam deflection technique can resolve deflections of up to 0.1 nm, implementation of this technique for reading in microcantilever arrays is a complex subject, such that there is no system, technique or method that allowed obtaining the response of several alignments of micro- and nanomechanical systems at different frequencies simultaneously. The present invention provides an optical microscopy technique based on the beam deflection method which simultaneously and automatically provides a spatial map of the static deflection and of the shape of five modes of vibration, with vertical resolution in the sub-angstrom range.